Effects of municipal sewage effluent on non-specific immune and thyroid functions of rainbow trout (Oncorhynchus mykiss) ABSTRACT Municipal sewage effluents contain complex mixtures of chemical and biological substances known to disrupt immunological functions in fish. Juvenile rainbow trout (Oncorhynchus mykiss) were exposed to concentrations of a primary-treated effluent for 45 d at 15°C in semi-static conditions. After the exposure period, the immune status – phagocytic activity, proliferation of B- and T-lymphocytes, and the ability to remove the pathogenic bacterium Aeromonas salmonicida were determined. In parallel, hepatic T 4 5′-deiodinase activity was determined. The results showed that the phagocytic response was bimodal; active phagocytosis was inhibited at the lowest concentration (0.003% v/v) but induced at higher concentrations (≥0.03% up to 3% v/v). The effluent inhibited proliferation of both T- and B-lymphocytes, but the inhibition was more severe for B-lymphocytes, suggesting that humoral immunity was more affected than cell-mediated immunity. The proportion of infected trout dropped in trout exposed to effluent compared to trout in aquarium water, suggesting that bacteria found in the effluent might have immunized fish against the pathogenic bacteria challenge. High K m hepatic T 4 5′-deiodinase activity was increased at one effluent concentration and was negatively correlated with active phagocytosis and proliferation of T-lymphocytes but was positively correlated with proliferation of B-lymphocytes. Hierarchical tree analysis of the data revealed that phagocytic activity was closely associated with proliferation of T-lymphocytes while B-lymphocyte activity was more closely correlated with high K m T 4 5′-deiodinase activity and the likelihood of developing an A. salmonicida infection. Municipal effluents have the potential to affect the thyroid and immune system of fish exposed for long periods at relatively low concentrations (≤3% v/v). Investigation of the immune status of feral fish populations and of the possible interplay between the immune system and thyroid status is warranted. KEYWORDS: deiodinase, immunotoxicity, municipal effluents, thyroid hormones, Oncorhynchus mykiss, rainbow trout, Aeromonas salmonicida INTRODUCTION Municipal wastewaters are complex mixtures of chemical and microbiological contaminants from an urban area which consists of commercial, institutional, and industrial sources, sanitary wastewater, underground infiltration units, and surface runoff. The municipal effluent from the city of Montreal (Quebec, Canada) is considered a major source of contamination of the 1 INRS-Institut Armand-Frappier, 245 boul. Hymus, Pointe-Claire, Quebec H9R 1G6, 2 Fluvial Ecosystem Research, Environment Canada, 105 McGill Street, Montreal, Quebec H2Y 2E7, 3 Ville de Montréal, Service des Infrastructures/Transport et Environnement, Direction de l’épuration des eaux usées, Ingénieries et Laboratoires, 12001 Maurice-Duplessis, Montreal, Quebec H1C 1V3, Canada Régina Escarné 1 , François Gagné 2, * , Claire Dautremepuits 1 , Daniel G. Cyr 1 , Kenneth Finnson 1 , David J. Marcogliese 2 , Jacques Bernier 1 , Patrick Cejka 3 and Michel Fournier 1 *Corresponding author francois.gagne@ec.gc.ca Trends in Comparative Biochem & Physiol Vol. 13, 2008